Caption: Rhyolitic geode. Cut surface of a geode displaying the host rock rhyolite (green), chalcedony (blue- white) and haematite (red). Geodes form when a hollow in a rock becomes filled with mineral crystals that grow inward to the centre of the hollow. Rhyolite is a volcanic rock formed by the cooling of lava on the Earth's surface. Chalcedony is a term for many cryptocrystalline quartz (silicon dioxide) gemstones, including agate. The colours are derived from impurities in the mineral. Haematite is a form of iron oxide. This geode came from the Esterel region on the Mediterranean coast in France.